Various types of sensors have been proposed for detecting the composition or flow of a gaseous vapor traveling through a duct. Very few, if any, can detect both composition and flow simultaneously and unambiguously. Sensors responsive to composition or flow have be based upon tin oxide, optical, hot wire and acoustical technologies.
Tin oxide sensors are nonlinear and quite sensitive at very low concentration levels only. Optical sensors include infrared absorption techniques. They have shown some promise in this area, but the technology has not yet been fully developed. Hot wire sensors respond simultaneously to gas composition and flow, without giving independent information on any of the two quantities. Acoustic type sensors may be designed to respond to gas composition alone, or to both gas composition and flow. To date, the spectral stability of their piezoelectric transducers over a wide temperature operating range is somewhat suspect.
As an alternative to using a single sensor to measure both composition and flow, multiple sensors can be utilized, each sensing a different parameter. For example, one sensing composition and another sensing flow. However, the use of multiple sensing technologies for measuring composition and flow independently has the inherent disadvantages of increasing the developmental, packaging and installation costs of the sensing system. Therefore, there is a desire to develop a single sensor which can measure both the composition and flow of gaseous vapors traveling through a duct. Typical applications would include automotive vehicle air/fuel monitoring and control, as well as gas tank canister purge control for meeting future vehicle emission requirements.